The invention relates to a piezoceramic multilayer actuator and to a method for its manufacture.
FIG. 1 shows a diagrammatic representation of a piezoceramic multilayer actuator 1 according to the prior art. These actuators feature an inter-digitated structure. They are manufactured as monoliths; i.e. they consist of stacked thin layers 2 of piezoelectrically active material, such as lead-zirconate-titanate (PZT), with conductive internal electrodes 7 arranged between them, which are led in an alternating manner to the surface of the actuator. The active material, as what is referred to as green film is provided before sintering, by means of a screen printing process with internal electrodes 7, is pressed into a stack, pyrolysed, and then sintered, as a result of which a monolithic multilayer actuator 1 is created.
External electrodes 3, 4 connect the internal electrodes 7. As a result of this, the internal electrodes 7 are, in each case, electrically connected in parallel on one side of the actuator 1, and thus combined in a group. The external electrodes 3, 4 are the connection poles of the actuator. If an electrical voltage is applied to the connection poles via the connections 5, it is transmitted in parallel onto all the internal electrodes 7, and produces an electrical field in all layers of the active material, which is consequently mechanically deformed. The sum total of all these mechanical deformations is available at the end surfaces of the actuator as a usable expansion 6 and/or force, for example for controlling an injection valve in combustion engines.
With actuators according to prior art, it is known that irregularities in expansion occur in the inactive region which is required for the purpose of contacting. These produce cracks in the piezoelectrically inactive electrode region, which can occur at regular intervals. This situation is represented in FIG. 1.
As a result of the expansion irregularities, the stresses in the passively expanded regions accumulate up to an amount at which they are relieved by the formation of cracks. The cracks 8 have, as a rule, spacings of more than one millimetre with expansions of some 1 to 2 μm/mm. These cracks 8 end in what is referred to as the active region of the actuator, which generates the expansion, since only pressure stresses occur at this point.
The cracks can be tolerated for a number of applications. There are, however, some fundamental problems. If the actuator is not fully encapsulated, electric fields occur at the electrode ends which are exposed by the cracks, which can lead to the accumulation of water or polar molecules. These cause leakage currents or lead to increased degradation of the actuator's performance. In addition to this, the possibility cannot be entirely excluded that the actuators, damaged beforehand by the cracks, will fail during operation because of fragmentation. In addition, the external electrodes are subjected to particular stress in the region of the cracks, and tear or come free. Failure of the external electrode leads to a total failure of an actuator.
The object of the invention is to eliminate as far as possible the causes of the occurrence of crack-forming stresses.
The solution of the object is achieved in accordance with the device and method according to the present invention.
A multilayer actuator in accordance with the invention differs from actuators of the prior art in that the surfaces for contacting with their external electrodes like the internal electrodes of same polarity, do not lie one above another in a straight direction. The internal electrodes of the multilayer actuator according to the invention have a contact region delimited by their base surface, tongue-shaped and pointing outwards. The front sides of these contact regions are the contact surfaces. Only these contact surfaces reach to the surface of the actuator and are connected to the respective external electrode in each case in order to connect the internal electrodes of the same polarity. Due to the aforementioned shape of the internal electrodes an inactive region with a width capable of being predetermined is formed in the tongue-shaped contact region. The contact surfaces, and therefore the inactive regions assigned to them, of one or a predetermined number of internal electrodes of the same polarity and arranged above one another in the same direction, are arranged offset by a specified angle a against the contact surfaces, and therefore the inactive regions assigned to them of the said internal electrodes, or a predetermined number of said internal electrodes aligned in the same direction, of the same polarity. As a result of this, inactive regions come into being, which are passively expanded.
The surfaces of the internal electrodes of the same polarity which are to be contacted are therefore arranged according to the invention at regular intervals offset at a predeterminable angle. The advantage in comparison with the prior art is that the stresses in the inactive regions can no longer total such a value that cracks occur.
After a predetermined amount, which is based on the size of the stresses occurring in this region, the offset should be so great, as a multiple of the angle α, that at least the contact surfaces of the first and last of the internal electrodes of the same polarity no longer overlap in this region. As a result, the critical distance in which the cracks form is overcome. Such offsets of a predetermined amount can be provided several times one behind another in a multilayer actuator, depending on its size, and may already be connected about every 0.5 mm, but should, as a function of the size of the actuator, be connected at the latest after about 3 mm, in order for the effect of the stress distribution to occur. An advantageous range is between 1 mm and 1.5 mm.
The thickness of a layer, i.e. the thickness of the piezoceramic layer and the thickness of the internal electrode, amounts, as a rule, to 100 μm, i.e. 0.1 mm. The offset at which the contact surfaces or the inactive regions no longer overlap, may occur as early as after about five layers, and should be concluded after about 30 layers. The region to be regarded as advantageous lies between about 10 and 15 layers. The offset to eliminate the covering of the contact surfaces and the inactive regions can be effected in such a way that each layer is arranged offset by a predetermined angle to the subsequent layer with an internal electrode of the same polarity. It is, however, also possible for two or more layers with internal electrodes of the same polarity initially to lie above one another in the same arrangement, before an offset is effected by a predetermined angle.
It is also possible for the offset to be continued, after the termination of one region or several regions, not only in one direction but back again in the opposite direction, so that a wave-shaped run of the offsets is formed. The application to the external electrode, however, is rendered somewhat more difficult as a result.
The actuators may feature a circular cross-section. The cross-section surface may however also be a square, a rectangle, or a polygon. When connecting the contact surfaces of internal electrodes of the same polarity on an actuator with circular cross-section and continuously zig-zagging offset, the external electrode has a helical-shaped progression. With the other design embodiments, an external electrode may extend over one side surface or, in particular with a polygonal cross-section, over several side surfaces.
For securing, the multilayer actuators may feature a continuous passage running along their longitudinal axes, with it being advantageous for these to be applied in the soft green bodies before sintering.
To reduce the stress in the inactive head and foot regions of the actuators, it is possible, by continuous reduction of the operating field strength, to form a uniform transition from active to inactive region. The reduction in the operating field strength is achieved by the distance between the internal electrodes being increased from electrode to electrode in the head and foot region, towards the respective end of the actuator in each case.
The invention is explained in greater detail on the basis of an exemplifying embodiment. The embodiment shows the structure of a multilayer actuator 10 with circular cross-section and continuous offset.